1. Field of the Invention
The present invention relates to an automatic transmission exclusively for an automobile used with torque converter, particularly to a structure of an automatic transmission mechanism.
2. Description of the Prior Art
An automatic transmission, in general, comprises two single planetary gear units, a sun gear which is used in common for said two single planetary gear units, a carrier of a first planetary gear unit and a ring gear of a second planetary gear unit which are connected together. The above structure is so-called simpson type in which driving force is input to a ring gear of said first planetary gear unit and/or said sun gear selectively, and said rotatory components of the planetary gear units are restrained by a brake or a one-way clutch, and output is taken from said ring gear of the second planetary gear unit which is coupled with said carrier of the first planetary gear unit. Then, forward three speeds are obtained.
A four speed automatic transmission is structured based on said forward three speed automatic transmission mechanism comprising two single planetary gear units. In addition to said three speed automatic transmission mechanism, by adding an over-drive mechanism or under-drive mechanism having one planetary gear unit, forward four speeds are obtained.
An automatic transmission mechanism adding an over-drive mechanism is axially long because an output member of an over-drive mechanism, i.e. a ring gear is connected axially by an input member of said three speeds automatic transmission.
On the other hand, said automatic transmission mechanism adding an over-drive mechanism has a counter drive gear (output) at the end of an output shaft ( farthest end from an input portion ) from which driiving force is transmitted to said under-drive mechanism placed under said three speed automatic transmission mechanism.
Recently, an automatic transmission has been required to make its mechanism small and to increase its horse power, however, said four speed automatic transmission having three planetary gear units with said over-drive mechanism can not satisfy such requirements.
Then, adding a clutch to connect or disconnect an input shaft and a carrier to obtain forward four speed is proposed, this addition of said clutch is for ravigneaux type planetary gear unit comprising a first sun gear, a second sun gear, a long pinion meshing said first sun gear, a short pinion meshing said long pinion and said second sun gear, a carrier supporting said long and said short pinion, and a ring gear meshing said long pinon.
The above said four speed automatic transmission can not satisfy a requirement to make its mechanism small because the structure of simpson type automatic tranmssion is axially long, i.e., driving force is input to one end and taken from the other end of mechanism and both gear units are separated.
Furthermore, for said ravigneaux type automatic transmission, comparing with that of simpson type, axial mechanical length can be shortened but the above requirements are not satisfied, yet, because ouput elements or a hydraulic pump are laid at the end of the shaft where replacement and modification can be done easily. Accordingly three speed and four speed of automatic transmission have to be manufactured respectively. Due to the above, it is difficult to manufacture parts, and assemble them in common for said three speed and four speed of transmission mechanisms, so, cost increase is inevitable, and such structure of an automatic transmission can not cope with recent wide variation trend.